Field: The instant invention relates to filtration techniques useful in filtering thin slurries of clay or mineral slimes, such as those derived from the mining of phosphate ores, bauxite, and talc or those involving in drilling muds by the addition of selected quantitative combinations of certain flocculating agents.
State of the Art: The dewatering of various types of mineral tailing slimes has been the subject of continual investigation since disposal represents a significant problem to various types of industries. Phosphate mining, for example, produces clay tailing slurries or slimes which have a low content of solids, for example, usually less than about 10%, which must be economically dewatered to a solids content of at least about 40% to be effectively returned to a mine site. The solids content of such slimes is composed of particles having an average particle size less than about 50 microns and often less than 25 microns. Slimes are generally characterized as aqueous suspensions of siliceous and clay solids including minor quantities of other minerals. Generally, slime dewatering techniques have yielded solids contents of only about 20%. Ponding has continued to be used to dispose of these tailing materials. Ponding, however, requires large acreages of land to dewater effectively the large volume of such slurries being produced. Because of environmental, economic, aesthetic and safety reasons, ponding is an undesirable approach to dewatering thin mineral slurries.
Recent work by the Bureau of Mines has investigated various flocculating agent combinations to increase the solids content of the dewatered slimes. Some of this work is described in Report of Investigations 8436, published in 1980 and entitled "Synergism and Polyethylene Oxide Dewatering of Phosphatic Clay Waste" by Smelley and Scheiner, which indicates that combinations of guar gums and polyethylene oxide (PEO) as a flocculating agent combination was superior over guar gum or PEO used singularly.
The Bureau of Mines report also referred to work with other combinations of PEO flocculating agents, with some results which indicated that synergism was apparently realized between PEO and an anionic polyacrylamide flocculating agent. The combination, however, apparently showed no superiority over PEO alone in terms of reagent economics.
The amount of guar required to reduce the requisite quantity of PEO necessary to achieve a flocculated slurry increased considerably when the PEO reduction was greater than about 50%. For example, at 2.5 pounds of guar per ton of processed slime, the PEO reduction was about 58%. An increase in the guar concentration to 5 pounds per ton reduced the PEO to 60%. However, at such a level, the cost of the guar per ton of processed slurry exceeded the cost of the polyethylene oxide when used at its usual concentration in the absence of guar.
Earlier work on the disposal problem of phosphate slimes is summarized in U.S. Pat. No. 3,680,698 of Liu et al. Liu et al. tried various coagulants by polymerizing one or more monomers in situ to form a particular polymer. While various polymers and copolymers were formed in situ by Liu et al., the use of a symbiotic or synergistic combination of distinct, separate polymers was not disclosed. The technique employed by Liu et al. produced a concentrated slime generally having a solids content of about 30% by volume (col. 5, lines 50 et seq.) although reference is made to concentrations up to 60% by weight solids (col. 6 lines 15 et seq.). The higher solids content is indicated to be achieved by mixing a sand slurry with a compacted product containing 10% or more solids.
The Liu et al. processing employed gravity dewatering apparatus, which has been conventionally used in slime processing in the phosphate industry. Such gravity separation devices are large, relatively slow operating apparatus requiring minimal attention and maintenance.
The Liu et al. coagulant system is intended to promote more rapid flocculation within these gravity water-solids separation apparatus which achieve separation of the solids from water by settling of the solid particles by agglomeration (flocculation) and settling.
While products containing in excess of 40% solids may be returned to a mine excavation as land fill, it is obviously desirable to increase the solids as high as possible while maintaining the use of flocculating agents in a range which is economic. Generally, at a solids content of 40%, mineral waste filter cakes are self-supporting.